Tech Review: FAST LSX, LSXR, and LSXRT Intake Manifolds

When the new LS engine first hit the streets, it was a big step forward for the small block Chevy family, introducing factory innovations like distributorless ignition, vastly improved cylinder head design (even when compared to the prior Vortec heads, which were themselves a quantum leap past previous factory castings), and a polymer intake manifold that didn’t retain and transfer heat like cast aluminum. Speaking of intakes, soon enough, LS1 hot rodders discovered there was a worthwhile bump in power from swapping on an OEM LS6 intake, but of course, there are always guys who want more.

Our manifolds are each designed to fit specific cylinder head types. – Matthew Maxwell

Fuel Air Spark Technology, better known simply as FAST, stepped in and offered the original LSX intake manifold, which gave cathedral-port engines some extra breathing room. The LSX logo appeared under the hood of a lot of very quick cars, serving the needs of both those who were looking for a bolt-on, plug and play high performance manifold, and those who were running hardcore (for the day, at least) mods and needed the best intake they could get, without going to the budget-busting expense of a bespoke fabricated race intake.

All FAST LSX Manifolds Feature:

  • Modular multi-piece design for easy internal access
  • Advanced polymer construction that won’t act as a heat sink like cast aluminum manifolds
  • Corrosion-free stainless steel assembly bolts
  • Improved throttle body sealing flange and O-ring
  • Threaded inserts for throttle body mounting
  • Provisions for factory emissions/fuel connections
  • Integrated nitrous bungs for direct port systems

As time passed, FAST introduced a new line of 102mm intakes under the LSXR designation, and quietly discontinued the original 92mm LSX manifold. Today, FAST has brought back the LSX, and between it and the variations in the LSXR line, pretty much the entire range of head designs – cathedral port, rectangular port, and raised rectangle – are all covered. So which FAST intake is right for your application, and what makes them the go-to manifold for so many engine builders? We dug into the design and engineering of these plastic fantastic manifolds to answer those questions.

To get the inside line on the development, testing, and manufacture of FAST’s LS intakes, we tapped Brian Reese, FAST’s VP of Product and Business Development, and Matthew Maxwell in Technical Sales & Support. Both were kind enough to field our questions, which ranged from the general to the esoteric.

One Size Doesn’t Fit All…

When the LS engine family was born, things were simple. Performance enthusiasts looking to help their LS1 or LS6 breathe easier didn’t have much choice when it came to manifolds, but FAST was on the job with one of the first aftermarket intakes. The original LSX manifold was designed to work with factory-style fuel rails and had all the plumbing provisions to make it a true “bolt on” swap. With the introduction of the LS3-style rectangular port cylinder head, and the raised-rectangle port LS7, FAST had to go back to the drawing board.

The new LSXR designs were about more than just going from ‘cathedral’ to rectangular ports, though – FAST took the opportunity to look at new options for the manifold’s internal layout, compared to the original LSX. One of the strengths of that manifold was that it was a “modular” three piece design, with the base, top, and runners all molded as separate pieces. When you open up the new LSXR family, you can see that they take things one step further.

One major change in the LSXR manifold lineup is the incorporation of individual intake runners rather than a single-piece runner molding for all eight cylinders.

“Yes, the new design is ‘modular’ with removable runners, a lower shell and upper shell,” explains Brian Reese. “There are multiple reasons why we planned this, but the three most notable are one, we can make unique runners suitable for performance objectives without having to redesign the whole manifold, two, we can cover all generations and applications of Gen III/IV LS engines with a single series of components, and three, we can make more power and torque with individual runners.”

The runners are retained in the lower shell with a screw, and seal against the manifold base using the same type of OEM-style O-ring gasket that seals the manifold to the head. Reese explains that the different LSXR runners share similar dimensions; “The removable section of runners are the same, but the lower runner and ports are different.  The geometry of the runners is actually very similar between LS3 and LS7, with the difference being related mostly to matching the head port location and shape.”

Internal Affairs

One of the benefits of the modular design is that it’s easy to port and modify these manifolds internally. We asked Reese what specific kinds of things FAST’s customers are doing to customize the LSX and LSXR manifolds to their needs. “Port matching is the most popular modification,” he explains. “Port matching customers are seeking incremental improvements by matching their particular head port. Port matching cannot change the tuning effects [of runner dimensions], though.”

We can make unique runners suitable for performance objectives without having to redesign the whole manifold. – Brian Reese

“We have some customers making custom runners or modifying stock runners,” Reese continues. “Moving the torque peak RPM band is the main motivation for making ‘other runners’.  Shorter is the only direction one can move though, because the stock FAST runners are as long as possible.” While the composite material’s properties and manufacturing methods mean that the designers at FAST have far more to work with than anyone pouring aluminum manifolds, there will always be those end users who want to experiment with some traditional means of modifying the shape or dimensions of the runners. Reese warns, “A proper epoxy could be used [to change runner shape] , but proceed with extreme caution.  Any failed adhesion could be broken loose and digested by the engine, resulting in failure.  We do not recommend this…”

The LSXRT manifold (left) is visibly taller than the LSXR, offering more space inside for longer, straighter runners.

Hitting New Heights

Another noteworthy member of the FAST LS manifold family is the LSXRT, where the little “T” stands for “Truck.” For use in applications where hood or cowl clearance isn’t an issue, this manifold has a few new tricks up its sleeve. “The LSXRT is dramatically different than the LSXR.  The plenum difference makes the majority of difference,” says Reese. “With the RT plenum, we have room to install straighter runners.  The LSXR runners will work and fit in the LSXRT, but they do not perform as well, due to the extra curvature.  The straightness impacted the size and taper as well; effectively the LSXrt is ‘smaller’ and this is the result of being straighter and less restrictive. In general terms, the LSXRT is the best-performing.”

Open up the LSXRT and the difference is even more pronounced. Shades of Can-Am!

Decisions, Decisions…

By now, you’ve probably decided you might just want a FAST manifold to bolt to the top of your LS. But which one? FAST offers what might be a daunting number of choices, but their online selection tool (and perhaps a little helpful advice from the tech staff) makes it straightforward. “Our manifolds are each designed to fit specific cylinder head types,” explains Matthew Maxwell. “Then there is truck manifold versus car manifold.  The toughest thing we have to deal with on the tech side is figuring out which injector the customer has and which fuel rail kit he will need.”

FAST offers a full range of fuel rail kits and large-diameter throttle bodies to match their LS intake manifolds.

One of the important differences between the LSX and LSXR/LSXRT is how they’re set up for fuel rails. Per Maxwell, “When the 102mm LSXR manifold was designed, the LS1/LS6 engines were history.  So the engineers decided to use the mounting locations of one of the current engines, the LS3. For manufacturing ease all the LSXR manifolds have the same top shell, and therefore the same fuel rail mounting locations.” While that was great for 5th Gen Camaro owners, guys with older LS engines needed to go through a bit more in order to have everything line up.

“The great thing about the 92mm intake is that LS1/LS6 injectors and fuel rails are direct fit parts,” Maxwell explains. During the time when the LSX was out of production, the LSXR was the only option from FAST. “This means the LS1/LS6 guys had to go out and buy new fuel rails to fit the new manifold. Having the 92mm LSX manifold back in the line makes swapping the manifold more of a “bolt-on.” Both Maxwell and Reese agree that if ultimate performance is paramount, the bigger 102mm LSXR is the way to go for cathedral port engines. “If all out horsepower is the goal, then the 102 is the way to go and changing fuel rails just goes with the territory,” Maxwell concludes.

All FAST LS manifolds incorporate provisions to drill and tap for direct port nitrous nozzles. What about boost? Per Reese, "The LSXR and LSXRT were specifically designed for up to 30 pounds of boost. We have guys running 20+ PSI without issue."

Maxwell adds one final helpful hint for Corvette owners: “The 102mm LSXR intake will fit on both the C5 and C6, but the 102 is pretty tight on firewall clearance on a C5 (about 5mm at the tightest point).  It does clear, but it is close.  The 92mm LSX, on the other hand, will absolutely NOT fit on an LS2-equipped C6 without firewall modification, though.”

FAST's LS manifolds all feature provisions for factory fuel, emissions, and sensor attachment, though some repositioning of the plumbing/wiring may be necessary. Threaded inserts molded into the shell make for trouble-free attachment.

Power to the People

FAST’s lineup of LS manifolds covers the vast majority of the street and racing spectrum, and most companies would be content to stop there. But FAST has gone the extra mile by offering 92 and 102mm throttle bodies, fuel rail kits, and accessories to make getting the right combination of parts a straightforward process. Combine that with their excellent online tech support and user forums, and you’re getting more than just molded composite – you’re getting the peace-of-mind that comes from knowing you’ve gotten the right advice to go along with the hardware.

About the author

Paul Huizenga

After some close calls on the street in his late teens and early twenties, Paul Huizenga discovered organized drag racing and never looked back, becoming a SFI-Certified tech inspector and avid bracket racer. Formerly the editor of OverRev and Race Pages magazines, Huizenga set out on his own in 2009 to become a freelance writer and editor.
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